Feature Request: Keep user defined tags

[c . buhtz]

Hello,

please allow me to suggest an improvement for gettext which would make 
my life easier.

I do have a po file like this

#, python-brace-format, my-foobar-tag
msgid "my id"
msgstr "meine ID"

My problem is that msgmerge --update create this out of it and remove 
the "my-foobar-tag" because it doesn't know it.

#, python-brace-format
msgid "my id"
msgstr "meine ID"

But I need to keep tags, even if msgmerge doesn't know them. There are 
good reason in my workflow to keep them.
For example I do run additional (semantic) tests on my po files. To 
influence the behavior of that tests, e.g. formulate exceptional cases, 
I use this tags.

Thanks in advance,
Christian Buhtz